1. Field of the Invention
The present invention relates to a substrate including an electrostatic discharge (ESD) protection function.
2. Description of the Related Art
ESD refers to a phenomenon in which an intense electrical discharge occurs when electrically charged conductive substances (e.g., a human body) contact or sufficiently approach other conductive substances (e.g., electronic devices). ESD causes problems, such as damage to electronic devices or malfunction thereof. In order to prevent these problems, an excessive voltage generated during discharge must be prevented from being applied to the circuits of electronic devices. For such a purpose, an ESD protection device is used, which is also referred to as a surge absorption element or a surge absorber.
The ESD protection device is disposed, for example, between a signal line and a ground of a circuit. The ESD protection device includes a pair of discharge electrodes that are arranged so as to face each other with a space therebetween, and the pair of electrodes has a high resistance in normal use and such that a signal does not flow to a ground side. In contrast, when an excessive voltage is applied, such as when static electricity is applied from an antenna of a cellular phone, for example, discharge occurs between the discharge electrodes of the ESD protection device, and thus, static electricity flows to the ground side. Thus, a voltage generated by static electricity is not applied to a circuit arranged downstream of the ESD device, and thus the circuit is protected.
For example, in an ESD protection device shown in an exploded perspective view of FIG. 18 and in a cross sectional view of FIG. 19, a cavity 5 is provided in a ceramic multilayer substrate 7 in which two or more insulating ceramic sheets 2 are laminated, discharge electrodes 6 electrically connected to external electrodes 1 are arranged to face each other in the cavity 5, and a discharge gas is enclosed in the cavity 5. When a voltage causing dielectric breakdown between the discharge electrodes 6 is applied, discharge occurs between the discharge electrodes 6 in the cavity 5. The discharge leads an excessive voltage to the ground, and thus, a circuit arranged downstream of the ESD protection device is protected (e.g., Japanese Unexamined Patent Application Publication No. 2001-43954).
However, when the ESD protection device is mounted, a space for the ESD protection device is required, which makes it difficult to reduce the size of a circuit. Moreover, the wiring distance from the ESD protection device to electronic circuits or electronic components to be protected is relatively long, and thus, the ESD protection performance is not sufficiently ensured because of the influence of wiring impedance.